Brain hyperthermia alters local cerebral glucose utilization: a comparison of hyperthermic agents

Cerebral Oxygen Delivery and Consumption in Brain-Injured Patients

Organism survival depends on oxygen delivery and utilization to maintain the balance of energy and toxic oxidants production. This regulation is crucial to the brain, especially after acute injuries. Secondary insults after brain damage may include impaired cerebral metabolism, ischemia, intracranial hypertension and oxygen concentration disturbances such as hypoxia or hyperoxia. Recent data highlight the important role of clinical protocols in improving oxygen delivery and resulting in lower mortality in brain-injured patients. Clinical protocols guide the rules for oxygen supplementation based on physiological processes such as elevation of oxygen supply (by mean arterial pressure (MAP) and intracranial pressure (ICP) modulation, cerebral vasoreactivity, oxygen capacity) and reduction of oxygen demand (by pharmacological sedation and coma or hypothermia). The aim of this review is to discuss oxygen metabolism in the brain under different conditions.

Nanodelivery of traditional Chinese Gingko Biloba extract EGb-761 and bilobalide BN-52021 induces superior neuroprotective effects on pathophysiology of heat stroke

Military personnel often exposed to high summer heat are vulnerable to heat stroke (HS) resulting in abnormal brain function and mental anomalies. There are reasons to believe that leakage of the blood-brain barrier (BBB) due to hyperthermia and development of brain edema could result in brain pathology. Thus, exploration of suitable therapeutic strategies is needed to induce neuroprotection in HS. Extracts of Gingko Biloba (EGb-761) is traditionally used in a variety of mental disorders in Chinese traditional medicine since ages. In this chapter, effects of TiO2 nanowired EGb-761 and BN-52021 delivery to treat brain pathologies in HS is discussed based on our own investigations. We observed that TiO2 nanowired delivery of EGb-761 or TiO2 BN-52021 is able to attenuate more that 80% reduction in the brain pathology in HS as compared to conventional drug delivery. The functional outcome after HS is also significantly improved by nanowired delivery of EGb-761 and BN-52021. These observations are the first to suggest that nanowired delivery of EGb-761 and BN-52021 has superior therapeutic effects in HS not reported earlier. The clinical significance in relation to the military medicine is discussed.

Mild Hyperthermia Aggravates Glucose Metabolic Consequences in Repetitive Concussion

Traumatic brain injury (TBI) is one of the leading causes of mortality and disability around the world. Mild TBI (mTBI) makes up approximately 80% of reported cases and often results in transient psychological abnormalities and cognitive disruption. At-risk populations for mTBI include athletes and other active individuals who may sustain repetitive concussive injury during periods of exercise and exertion when core temperatures are elevated. Previous studies have emphasized the impact that increased brain temperature has on adverse neurological outcomes. A lack of diagnostic tools to assess concussive mTBI limits the ability to effectively identify the post-concussive period during which the brain is uniquely susceptible to damage upon sustaining additional injury. Studies have suggested that a temporal window of increased vulnerability that exists corresponds to a period of injury-induced depression of cerebral glucose metabolism. In the current study, we sought to evaluate the relationship between repetitive concussion, local cerebral glucose metabolism, and brain temperature using the Marmarou weight drop model to generate mTBI. Animals were injured three consecutive times over a period of 7 days while exposed to either normothermic or hyperthermic temperatures for 15 min prior to and 1 h post each injury. A ¹⁴C-2-deoxy-d-glucose (2DG) autoradiography was used to measure local cerebral metabolic rate of glucose (lCMR_Glc) in 10 diverse brain regions across nine bregma levels 8 days after the initial insult. We found that repetitive mTBI significantly decreased glucose utilization bilaterally in several cortical areas, such as the cingulate, visual, motor, and retrosplenial cortices, as well as in subcortical areas, including the caudate putamen and striatum, compared to sham control animals. lCMR_Glc was significant in both normothermic and hyperthermic repetitive mTBI animals relative to the sham group, but to a greater degree when exposed to hyperthermic conditions. Taken together, we report significant injury-induced glucose hypometabolism after repetitive concussion in the brain, and additionally highlight the importance of temperature management in the acute period after brain injury.

Cerebral Vascular Control and Metabolism in Heat Stress

This review provides an in-depth update on the impact of heat stress on cerebrovascular functioning. The regulation of cerebral temperature, blood flow, and metabolism are discussed. We further provide an overview of vascular permeability, the neurocognitive changes, and the key clinical implications and pathologies known to confound cerebral functioning during hyperthermia. A reduction in cerebral blood flow (CBF), derived primarily from a respiratory-induced alkalosis, underscores the cerebrovascular changes to hyperthermia. Arterial pressures may also become compromised because of reduced peripheral resistance secondary to skin vasodilatation. Therefore, when hyperthermia is combined with conditions that increase cardiovascular strain, for example, orthostasis or dehydration, the inability to preserve cerebral perfusion pressure further reduces CBF. A reduced cerebral perfusion pressure is in turn the primary mechanism for impaired tolerance to orthostatic challenges. Any reduction in CBF attenuates the brain's convective heat loss, while the hyperthermic-induced increase in metabolic rate increases the cerebral heat gain. This paradoxical uncoupling of CBF to metabolism increases brain temperature, and potentiates a condition whereby cerebral oxygenation may be compromised. With levels of experimentally viable passive hyperthermia (up to 39.5-40.0 °C core temperature), the associated reduction in CBF (∼ 30%) and increase in cerebral metabolic demand (∼ 10%) is likely compensated by increases in cerebral oxygen extraction. However, severe increases in whole-body and brain temperature may increase blood-brain barrier permeability, potentially leading to cerebral vasogenic edema. The cerebrovascular challenges associated with hyperthermia are of paramount importance for populations with compromised thermoregulatory control--for example, spinal cord injury, elderly, and those with preexisting cardiovascular diseases.

Alleviating effect of Mallotus roxburghianus in heat-induced testicular dysfunction in Wistar rats

CONTEXT

Hyperthermia causes detrimental effects on the testes leading to fertility problems. Mallotus roxbhurghianus Muell. Arg. (Euphorbiaceae) is used in traditional medicine and possesses antioxidant property. However, the mechanisms remain unknown in the context of alleviative action of M. roxburghianus against heat stress.

OBJECTIVE

The objective of this study was to demonstrate the alleviating activity of M. roxburghianus and its mechanism in scrotal hyperthermia.

MATERIALS AND METHODS

Scrotal hyperthermia experiments were performed in three groups (n = 7 per group) consisting of (i) the control group (C) maintained at 22 °C for 30 min, (ii) the heat stress-induced group (HS), and (iii) the heat stress-induced M. roxburghianus-treated group (HSM - 400 mg/kg each) in a thermostatically controlled water bath at 43 °C for 30 min. Subsequent to the heat treatment HS group, rats were treated with saline p.o and methanol extract of M. roxburghianus was administered to the rats of HSM group along with their standard food for 14 d. Scrotal hyperthermic effects were evaluated.

RESULTS

Scrotal hyperthermia significantly (p < 0.0001) elevated malondialdehyde levels while decreasing the body and testes weights, serum testosterone, and antioxidant enzyme levels due to oxidative stress. Disorganisation of seminiferous tubules and arrest of spermatogenesis were observed in the HS group. The administration of methanol extract of M. roxburghianus (400 mg/kg) for 14 d after heat treatment significantly suppressed the lipid peroxidation, restored the antioxidant enzyme and testosterone levels, revived the spermatogenesis, and increased the cell proliferation activity in the HSM group.

DISCUSSION AND CONCLUSION

The methanol extract of M. roxburghianus accelerates testicular recovery from the damaging influence of hyperthermia.

NIR-driven Smart Theranostic Nanomedicine for On-demand Drug Release and Synergistic Antitumour Therapy

Smart nanoparticles (NPs) that respond to external and internal stimulations have been developing to achieve optimal drug release in tumour. However, applying these smart NPs to attain high antitumour performance is hampered by limited drug carriers and inefficient spatiotemporal control. Here we report a noninvasive NIR-driven, temperature-sensitive DI-TSL (DOX/ICG-loaded temperature sensitive liposomes) co-encapsulating doxorubicin (DOX) and indocyanine green (ICG). This theranostic system applies thermo-responsive lipid to controllably release drug, utilizes the fluorescence (FL) of DOX/ICG to real-time trace the distribution of NPs, and employs DOX/ICG to treat cancer by chemo/photothermal therapy. DI-TSL exhibits uniform size distribution, excellent FL/size stability, enhanced response to NIR-laser, and 3 times increased drug release through laser irradiation. After endocytosis by MCF-7 breast adenocarcinoma cells, DI-TSL in cellular endosomes can cause hyperthermia through laser irradiation, then endosomes are disrupted and DI-TSL ‘opens’ to release DOX simultaneously for increased cytotoxicity. Furthermore, DI-TSL shows laser-controlled release of DOX in tumour, enhanced ICG and DOX retention by 7 times and 4 times compared with free drugs. Thermo-sensitive DI-TSL manifests high efficiency to promote cell apoptosis, and completely eradicate tumour without side-effect. DI-TSL may provide a smart strategy to release drugs on demand for combinatorial cancer therapy.

Cerebral oxygenation and hyperthermia

Hyperthermia is associated with marked reductions in cerebral blood flow (CBF). Increased distribution of cardiac output to the periphery, increases in alveolar ventilation and resultant hypocapnia each contribute to the fall in CBF during passive hyperthermia; however, their relative contribution remains a point of contention, and probably depends on the experimental condition (e.g., posture and degree of hyperthermia). The hyperthermia-induced hyperventilatory response reduces arterial CO₂ pressure (PaCO₂) causing cerebral vasoconstriction and subsequent reductions in flow. During supine passive hyperthermia, the majority of recent data indicate that reductions in PaCO₂ may be the primary, if not sole, culprit for reduced CBF. On the other hand, during more dynamic conditions (e.g., hemorrhage or orthostatic challenges), an inability to appropriately decrease peripheral vascular conductance presents a condition whereby adequate cerebral perfusion pressure may be compromised secondary to reductions in systemic blood pressure. Although studies have reported maintenance of pre-frontal cortex oxygenation (assessed by near-infrared spectroscopy) during exercise and severe heat stress, the influence of cutaneous blood flow is known to contaminate this measure. This review discusses the governing mechanisms associated with changes in CBF and oxygenation during moderate to severe (i.e., 1.0°C to 2.0°C increase in body core temperature) levels of hyperthermia. Future research directions are provided.

Brain temperature: physiology and pathophysiology after brain injury

The regulation of brain temperature is largely dependent on the metabolic activity of brain tissue and remains complex. In intensive care clinical practice, the continuous monitoring of core temperature in patients with brain injury is currently highly recommended. After major brain injury, brain temperature is often higher than and can vary independently of systemic temperature. It has been shown that in cases of brain injury, the brain is extremely sensitive and vulnerable to small variations in temperature. The prevention of fever has been proposed as a therapeutic tool to limit neuronal injury. However, temperature control after traumatic brain injury, subarachnoid hemorrhage, or stroke can be challenging. Furthermore, fever may also have beneficial effects, especially in cases involving infections. While therapeutic hypothermia has shown beneficial effects in animal models, its use is still debated in clinical practice. This paper aims to describe the physiology and pathophysiology of changes in brain temperature after brain injury and to study the effects of controlling brain temperature after such injury.

Temperature patterns in the early postresuscitation period after pediatric inhospital cardiac arrest

OBJECTIVE

To describe the prevalence of postarrest hyperthermia among children during the first 24 hrs after inhospital cardiac arrest and to determine the association of persistent postarrest hyperthermia with neurologic outcome and death before hospital discharge.

DESIGN

Multicenter, national registry of inhospital cardiopulmonary resuscitation.

SETTING

A total of 196 hospitals reporting to the American Heart Association's National Registry of Cardiopulmonary Resuscitation from January 1, 2005 to December 31, 2007.

PATIENTS

A total of 547 pediatric patients who suffered inhospital pulseless cardiac arrests reported to the National Registry of Cardiopulmonary Resuscitation, who survived resuscitative efforts and who had the maximum and the minimum temperature in the first 24 hrs postresuscitation reported to the National Registry of Cardiopulmonary Resuscitation.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Among 547 children with pulseless cardiac arrests, 238 (43.5%) had at least one temperature of ≥38°C, and 30 (5.5%) had "persistent hyperthermia" (i.e., both the minimum and the maximum temperature of ≥38°C) during the first 24 hrs postarrest. After adjusting for potential confounders by multivariate logistic regression, persistent hyperthermia in the first 24 hrs postarrest was associated with unfavorable neurologic outcome (adjusted odds ratio, 2.7; 95% confidence interval, 1.1-6.7), but not with death before hospital discharge (adjusted odds ratio, 1.2; 95% confidence interval, 0.4-3.4).

CONCLUSIONS

Despite current guidelines to avoid postarrest hyperthermia, a temperature of ≥38°C occurred commonly among children in the first 24 hrs postarrest. Persistent postarrest hyperthermia was associated with unfavorable neurologic outcomes, even after controlling for potential confounding factors.

Cerebral pathophysiology and clinical neurology of hyperthermia in humans

Deliberate hyperthermia has been used clinically as experimental therapy for neoplastic and infectious diseases. Several case fatalities have occurred with this form of treatment, but most were attributable to systemic complications rather than central nervous system toxicity. Nonetheless, demyelating peripheral neuropathy and neurological symptoms of nausea, delirium, apathy, stupor, and coma have been reported. Temperatures exceeding 40 degrees C cause transient vasoparalysis in humans, resulting in cerebral metabolic uncoupling and loss of pressure-flow autoregulation. These findings may be related to the development of brain edema, intracerebral hemorrhage, and intracranial hypertension observed after prolonged therapeutic hyperthermia. Furthermore, deliberate hyperthermia critically worsens the extent of histopathological damage in animal models of traumatic, ischemic, and hypoxic brain injury. However, it is unknown whether these findings translate to episodes of spontaneous fever in neurologically injured patients. In a clinical setting fever is a strong prognostic marker of a patient's primary degree of neuronal damage, and a causal relation with long-term functional neurological outcome has not been established for most types of brain injury. Furthermore, in the neurosurgical intensive-care unit fever is extremely common whereas antipyretic therapy is only poorly effective. Therefore maintaining strict normothermia may be an impossible goal in many patients. Although there are several physiological arguments for avoiding exogenous hyperthermia in neurologically injured patients, there is no evidence that aggressive attempts at controlling spontaneous fever can improve clinical outcome.

Theophylline attenuates microwave-induced impairment of memory acquisition

Numerous studies have shown that acute microwave exposure causes cognitive deficits in animals, possibly via hyperthermia, but the biological effect of microwave exposure on memory processing is still unknown. The release of adenosine is demonstrated to be a general way for the cells to respond to metabolically stressful conditions such as hypoxia and ischemia. The present study aimed to examine whether adenosine mediates biological effects of microwave exposure on memory processing using a continuous multiple-trial inhibitory avoidance task. Results demonstrated that microwave exposure for 20 min before training impaired memory acquisition and retention performance in mice, assessed by the number of training trials and by latency to enter the dark compartment. The mice exposed to microwave radiation showed a dose-dependent hyperthermia. Moreover, the cell numbers of hippocampus were decreased in the mice receiving microwave exposure at an average power density of 50 mW/cm(2), indicating the anatomical correlation to hippocampal-amygdaloid structures corresponding with the memory disrupt of the mice. Administration of theophylline, a nonspecific adenosine receptor antagonist, 30 min before microwave exposure, completely antagonized the impairment of inhibitory avoidance acquisition but not retention. These results suggest that the adenosine regulation pathway was partially involved in microwave-induced impairment of inhibitory avoidance memory.

Impact of pyrexia on neurochemistry and cerebral oxygenation after acute brain injury

BACKGROUND

Postischaemic pyrexia exacerbates neuronal damage. Hyperthermia related cerebral changes have still not been well investigated in humans.

OBJECTIVE

To study how pyrexia affects neurochemistry and cerebral oxygenation after acute brain injury.

METHODS

18 acutely brain injured patients were studied at the onset and resolution of febrile episodes (brain temperature > or = 38.7 degrees C). Intracranial pressure (ICP), brain tissue oxygen tension (PbrO2), and brain tissue temperature (Tbr) were recorded continuously; jugular venous blood was sampled intermittently. Microdialysis probes were inserted in the cerebral cortex and in subcutaneous tissue. Glucose, lactate, pyruvate, and glutamate were measured hourly. The lactate to pyruvate ratio was calculated.

RESULTS

Mean (SD) Tbr rose from 38 (0.5) to 39.3 (0.3) degrees C. Arteriojugular oxygen content difference (AJD(O2)) fell from 4.2 (0.7) to 3.8 (0.5) vol% (p < 0.05) and PbrO2 rose from 32 (21) to 37 (22) mm Hg (p < 0.05). ICP increased slightly and no significant neurochemical alterations occurred. Opposite changes were recorded when brain temperature returned towards baseline.

CONCLUSIONS

As long as substrate and oxygen delivery remain adequate, hyperthermia on its own does not seem to induce any further significant neurochemical alterations. Changes in cerebral blood volume may, however, affect intracranial pressure.

Imaging brain phospholipase A2-mediated signal transduction in response to acute fluoxetine administration in unanesthetized rats

Fluoxetine, a selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor, is used widely to treat depression and related disorders. By inhibiting presynaptic 5-HT reuptake, fluoxetine is thought to act by increasing 5-HT in the synaptic cleft, thus 5-HT binding to postsynaptic 5-HT(2A/2C) receptors. These receptors can be coupled via a G-protein to phospholipase A(2) (PLA(2)), which when activated releases the second messenger arachidonic acid from synaptic membrane phospholipids. To image this activation, fluoxetine (10 mg/kg) or saline vehicle was administered i.p. to unanesthetized rats, and regional brain incorporation coefficients k(*) of intravenously injected radiolabeled arachidonic acid were measured after 30 min. Compared with vehicle, fluoxetine significantly increased k(*) in prefrontal, motor, somatosensory, and olfactory cortex, as well as in the basal ganglia, hippocampus, and thalamus. Many of these regions demonstrate high densities of the serotonin reuptake transporter and of 5-HT(2A/2C) receptors. Brain stem, spinal cord, and cerebellum, which showed no significant response to fluoxetine, have low densities of the transporters and receptors. The results show that it is possible to image quantitatively PLA(2)-mediated signal transduction in vivo in response to fluoxetine.

Effects of hyperthermia on cerebral blood flow and metabolism during prolonged exercise in humans

The development of hyperthermia during prolonged exercise in humans is associated with various changes in the brain, but it is not known whether the cerebral metabolism or the global cerebral blood flow (gCBF) is affected. Eight endurance-trained subjects completed two exercise bouts on a cycle ergometer. The gCBF and cerebral metabolic rates of oxygen, glucose, and lactate were determined with the Kety-Schmidt technique after 15 min of exercise when core temperature was similar across trials, and at the end of exercise, either when subjects remained normothermic (core temperature = 37.9 degrees C; control) or when severe hyperthermia had developed (core temperature = 39.5 degrees C; hyperthermia). The gCBF was similar after 15 min in the two trials, and it remained stable throughout control. In contrast, during hyperthermia gCBF decreased by 18% and was therefore lower in hyperthermia compared with control at the end of exercise (43 +/- 4 vs. 51 +/- 4 ml. 100 g(-1). min(-1); P < 0.05). Concomitant with the reduction in gCBF, there was a proportionally larger increase in the arteriovenous differences for oxygen and glucose, and the cerebral metabolic rate was therefore higher at the end of the hyperthermic trial compared with control. The hyperthermia-induced lowering of gCBF did not alter cerebral lactate release. The hyperthermia-induced reduction in exercise cerebral blood flow seems to relate to a concomitant 18% lowering of arterial carbon dioxide tension, whereas the higher cerebral metabolic rate of oxygen may be ascribed to a Q(10) (temperature) effect and/or the level of cerebral neuronal activity associated with increased exertion.

Changes in regional cerebral metabolism during systemic hyperthermia in humans

Whole body hyperthermia may produce vasodialation, nausea, and altered cognitive function. Animal research has identified brain regions that have important roles in thermoregulation. However, differences in both the cognitive and sweating abilities of humans and animals implicate the need for human research. Positron emission tomography (PET) was used to identify brain regions with altered activity during systemic hyperthermia. Human subjects were studied under cool (control) conditions and during steady-state hyperthermia induced by means of a liquid-conditioned suit perfused with hot water. PET images were obtained by injecting [(18)F]fluorodeoxyglucose, waiting 20 min for brain uptake, and then scanning for 10 min. Heating was associated with a 23% increase in resting metabolic rate. Significant increases in cerebral metabolic rate occurred in the hypothalamus, thalamus, corpus callosum, cingulate gyrus, and cerebellum. In contrast, significant decreases occurred in the caudate, putamen, insula, and posterior cingulum. These results are important for understanding the mechanisms responsible for altered cognitive and systemic responses during hyperthermia. Novel regions (e.g., lateral cerebellum) with possible thermoregulatory roles were identified.